Field of the Invention
The present invention relates to a method for efficiently removing an organic extraction solvent from an extraction residue while the residue is in an oil-and-fat extracting apparatus (or oil-and-fat extractor), so that, upon malfunction of the extractor, the concentration of the solvent in the atmosphere of the extractor can be reduced to a level lower than the minimum ignitable limit.
Discussion of the Background
In an oil-and-fat extractor, oilseed such as soybean, rapeseed, corn, sunflower seed, cotton seed and the like, are separated into (i) oil and fat and (ii) residue, by using an organic extracting agent, for example, hexane. Various types of extractors have been used for the separation, including Rotocel.RTM., Lurgi.RTM., De Smet.RTM. and Crown.RTM. extractors. In principle, these extractors work in a similar manner and are capable of separating oil-and-fat from oil seeds.
There are cases where an extractor stops due to mechanical trouble during the course of extraction, thereby making it impossible to draw out the extraction residue remaining in the extractor. In order to rectify the trouble, a person may have to enter the interior of the extracton. However, if a high concentration of organic solvent is contained in the atmosphere of the extractor, it may cause fire or explosion even when no fire is used in the extractor since fire or explosion can be caused by an electric spark generated by static charge or an accidental contact or collision between metals. Therefore, the concentration of solvent in the atmosphere of the extractor must be reduced to a level lower than the lowest ignitable limit. Organic solvents which are not contained in the residue can be drawn out of the extractor with no difficulty. However, it can be troublesome to draw out organic solvents contained in the extraction residue in a reliable and effective manner.
Heretofore, the removal of organic solvents contained in the extraction residue has been effected by passing air or blowing steam through the residue. An extremely long period of time is required to remove organic solvents contained in the extraction residue according to these prior methods. For example, when air was passed through an extractor of a capacity of 60 tons for a period of 3 days to remove organic solvents through diffusion, the layer of extraction residue still formed a detonatable gas. Even in the case of steam blowing, it is difficult to remove the organic solvent from the layer of extraction residue in a uniform manner within a short period of time. Furthermore, the organic solvent, if released into the atmosphere, could cause air pollution. It has therefore been desired to develop a method which enables one to remove a large quantity of organic solvent remaining in the extractor in a simple manner within a short period of time.